Add tags chapter

book/04-events.md

@@ -38,8 +38,16 @@ use sha2::{Digest, Sha256};
 use std::fmt;
 
 use crate::keys::PublicKey;
+use crate::tags::Tags;
 ```
 
+The `Tag` and `Tags` types are introduced in the next chapter. For
+this chapter, treat `Tag` as a transparent wrapper around
+`Vec<String>` and `Tags` as a transparent wrapper around `Vec<Tag>` —
+serde sees them as bare arrays, the canonical form hashes identically,
+and you can build them with `Tag::new("t", ["nostr"])` and
+`Tags::from(vec![...])`.
+
 ## Errors
 
 ```rust {file=coracle-lib/src/events.rs}
@@ -104,12 +112,12 @@ metadata in the "could be added later" sense.
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct EventContent {
     pub content: String,
-    pub tags: Vec<Vec<String>>,
+    pub tags: Tags,
 }
 
 impl EventContent {
-    pub fn new(content: impl Into<String>, tags: Vec<Vec<String>>) -> Self {
-        EventContent { content: content.into(), tags }
+    pub fn new(content: impl Into<String>, tags: impl Into<Tags>) -> Self {
+        EventContent { content: content.into(), tags: tags.into() }
     }
 }
 ```
@@ -126,7 +134,7 @@ which kinds mean what — that's the next layer of the stack.
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct EventTemplate {
     pub content: String,
-    pub tags: Vec<Vec<String>>,
+    pub tags: Tags,
     pub kind: u16,
 }
 
@@ -151,7 +159,7 @@ it is.
 pub struct StampedEvent {
     pub content: String,
     pub kind: u16,
-    pub tags: Vec<Vec<String>>,
+    pub tags: Tags,
     pub created_at: u64,
 }
 
@@ -179,7 +187,7 @@ claims responsibility for.
 pub struct OwnedEvent {
     pub content: String,
     pub kind: u16,
-    pub tags: Vec<Vec<String>>,
+    pub tags: Tags,
     pub created_at: u64,
     pub pubkey: PublicKey,
 }
@@ -216,7 +224,7 @@ fn canonical(
     pubkey: &PublicKey,
     created_at: u64,
     kind: u16,
-    tags: &[Vec<String>],
+    tags: &Tags,
     content: &str,
 ) -> String {
     serde_json::json!([
@@ -235,7 +243,7 @@ fn canonical(
 pub struct HashedEvent {
     pub content: String,
     pub kind: u16,
-    pub tags: Vec<Vec<String>>,
+    pub tags: Tags,
     pub created_at: u64,
     pub pubkey: PublicKey,
     pub id: [u8; 32],
@@ -271,7 +279,7 @@ wire.
 pub struct Event {
     pub content: String,
     pub kind: u16,
-    pub tags: Vec<Vec<String>>,
+    pub tags: Tags,
     pub created_at: u64,
     pub pubkey: PublicKey,
     pub id: [u8; 32],
@@ -423,7 +431,7 @@ impl<'de> Visitor<'de> for EventVisitor {
         let mut pubkey: Option<String> = None;
         let mut created_at: Option<u64> = None;
         let mut kind: Option<u16> = None;
-        let mut tags: Option<Vec<Vec<String>>> = None;
+        let mut tags: Option<Tags> = None;
         let mut content: Option<String> = None;
         let mut sig: Option<String> = None;
 

book/05-tags.md

@@ -0,0 +1,267 @@
+# Tags
+
+Every nostr event has two halves. The `content` string is for humans:
+the text of a note, the prose of an article, the caption of an image.
+Everything else that matters to a machine — who this event replies to,
+which topics it belongs under, which pubkeys it mentions, which relay
+the author recommends, which article it updates — lives in `tags`.
+
+A tag is a list of strings. The first string names the tag; the rest
+are its values. An event's `tags` field is a list of these. That's the
+whole definition:
+
+```text
+["e", "4376c65d...", "wss://relay.example", "reply"]
+["p", "6e468422..."]
+["a", "30023:6e468422...:my-article-slug"]
+["t", "nostr"]
+```
+
+It looks plainer than it is. Three choices in that shape matter.
+
+**Lists of lists, not maps.** If tags were a dictionary, a key could
+only appear once and the order would be lost. Neither property is one
+nostr can give up. An event commonly references several pubkeys
+(`["p", ...]` repeated), several events (`["e", ...]` repeated), and
+several topics (`["t", ...]` repeated), and the order these appear in
+sometimes carries meaning — NIP-10 reply threads, for instance, use
+position as a fallback when explicit markers are missing. Lists of
+lists preserve both.
+
+**Single-letter names are indexed.** Relays index tags whose name is a
+single letter (`a` through `z`, `A` through `Z`) and let clients query
+them with `#e`, `#p`, `#t` filters. Multi-character names — `alt`,
+`imeta`, `expiration` — are carried on the wire but not indexed. The
+distinction matters at design time: if you want something to be
+queryable, give it a single-letter name.
+
+**Meaning is kind-dependent.** The `e` tag appears in eight different
+NIPs and means eight different things: a reply, a fork, a merge, a
+transaction reference, a report target, a list member, an approval,
+a mention. There is no way to interpret `["e", ...]` without first
+knowing the event's kind. The [building-nostr] philosophy puts this
+bluntly: "when resolving the meaning of a tag, always first look at
+the specifications for the event's kind." A library type that tries
+to parse tags into a taxonomy is betting on the wrong end of that
+rule.
+
+This chapter therefore introduces a type that is almost nothing. `Tag`
+wraps `Vec<String>` and adds five or six convenience methods. A
+handful of free functions query a slice of tags by name. That is
+enough for every caller we'll meet in the rest of the book. The one
+exception is the address tag, `a`, whose payload is three fields
+glued with colons — that gets a small `Address` struct, because those
+three fields always travel together and parsing them at the edge of
+your program is genuinely useful.
+
+[building-nostr]: https://building-nostr.coracle.social
+
+## The module
+
+```rust {file=coracle-lib/src/lib.rs}
+pub mod tags;
+```
+
+```rust {file=coracle-lib/src/tags.rs}
+//! The nostr `Tag` type: a thin wrapper around `Vec<String>` with
+//! accessors and a set of free functions that query slices of tags
+//! by name.
+
+use serde::{Deserialize, Serialize};
+```
+
+## The `Tag` type
+
+A `Tag` is a `Vec<String>`. We wrap it as a tuple struct so that it
+has its own type name and its own set of methods, and we mark the
+serde impl `transparent` so that on the wire — and in the canonical
+hash bytes — it is indistinguishable from the raw array.
+
+```rust {file=coracle-lib/src/tags.rs}
+/// A single nostr tag: the first string is the tag's name, the rest
+/// are its values.
+///
+/// `Tag` serializes transparently as its inner `Vec<String>`, so the
+/// wire format and the canonical hash bytes are unchanged from a bare
+/// `Vec<Vec<String>>`. Wrapping it in a type only exists to hang
+/// accessors and helpers off of, and to give call sites something to
+/// read.
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(transparent)]
+pub struct Tag(pub Vec<String>);
+```
+
+Most of the methods on `Tag` just peek at positions in the vector. An
+empty tag is technically well-formed but meaningless, so `name` and
+`value` return `""` rather than panicking or returning an `Option`;
+readers almost always want to compare against a known string and the
+empty-string fallback makes those comparisons safe.
+
+```rust {file=coracle-lib/src/tags.rs}
+impl Tag {
+    /// Build a tag from a name and a list of values.
+    ///
+    /// ```
+    /// # use coracle_lib::tags::Tag;
+    /// let t = Tag::new("t", ["nostr"]);
+    /// assert_eq!(t.name(), "t");
+    /// assert_eq!(t.value(), "nostr");
+    /// ```
+    pub fn new<N, V, S>(name: N, values: V) -> Self
+    where
+        N: Into<String>,
+        V: IntoIterator<Item = S>,
+        S: Into<String>,
+    {
+        let mut v = Vec::new();
+        v.push(name.into());
+        v.extend(values.into_iter().map(Into::into));
+        Tag(v)
+    }
+
+    /// The tag's name — its first entry, or `""` if the tag is empty.
+    pub fn name(&self) -> &str {
+        self.0.first().map(String::as_str).unwrap_or("")
+    }
+
+    /// The tag's primary value — its second entry, or `""` if absent.
+    pub fn value(&self) -> &str {
+        self.0.get(1).map(String::as_str).unwrap_or("")
+    }
+
+    /// Every entry after the name.
+    pub fn values(&self) -> &[String] {
+        if self.0.is_empty() { &[] } else { &self.0[1..] }
+    }
+
+    /// The entry at `i`, or `None` if out of bounds.
+    pub fn get(&self, i: usize) -> Option<&str> {
+        self.0.get(i).map(String::as_str)
+    }
+
+    /// The number of entries in the tag (name plus values).
+    pub fn len(&self) -> usize {
+        self.0.len()
+    }
+
+    /// Whether the tag is empty.
+    pub fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+
+    /// Borrow the underlying slice.
+    pub fn as_slice(&self) -> &[String] {
+        &self.0
+    }
+}
+```
+
+Conversions to and from `Vec<String>` make it painless to drop into
+and out of the wrapper:
+
+```rust {file=coracle-lib/src/tags.rs}
+impl From<Vec<String>> for Tag {
+    fn from(v: Vec<String>) -> Self {
+        Tag(v)
+    }
+}
+
+impl From<Tag> for Vec<String> {
+    fn from(t: Tag) -> Self {
+        t.0
+    }
+}
+```
+
+## A collection of tags
+
+When you hold an `Event`, you usually want to ask questions of its
+`tags` field: does it have a `p` tag? What's its `d` value? Which
+topics is it tagged with? These come up often enough that writing the
+filter by hand every time is noise. They belong on a type.
+
+We introduce `Tags` as a newtype around `Vec<Tag>`. Like `Tag` itself,
+the serde impl is transparent, so the wire format and canonical hash
+bytes are unchanged. A `Deref<Target = [Tag]>` impl gives iteration,
+indexing, and `len` for free, so the wrapper costs almost nothing at
+call sites.
+
+```rust {file=coracle-lib/src/tags.rs}
+/// A collection of tags, usually taken from an [`Event`]'s `tags`
+/// field. Serializes transparently as its inner `Vec<Tag>`.
+#[derive(Debug, Clone, PartialEq, Eq, Default, Serialize, Deserialize)]
+#[serde(transparent)]
+pub struct Tags(pub Vec<Tag>);
+
+impl Tags {
+    /// An empty collection.
+    pub fn new() -> Self {
+        Tags(Vec::new())
+    }
+
+    /// Return the first tag with the given name.
+    pub fn find(&self, name: &str) -> Option<&Tag> {
+        self.0.iter().find(|t| t.name() == name)
+    }
+
+    /// Iterate over every tag with the given name.
+    pub fn find_all<'a>(&'a self, name: &'a str) -> impl Iterator<Item = &'a Tag> + 'a {
+        self.0.iter().filter(move |t| t.name() == name)
+    }
+
+    /// Return the value (second entry) of the first tag with the
+    /// given name, or `None` if no such tag exists.
+    pub fn value(&self, name: &str) -> Option<&str> {
+        self.find(name).map(Tag::value)
+    }
+
+    /// Iterate over the values of every tag with the given name.
+    pub fn values<'a>(&'a self, name: &'a str) -> impl Iterator<Item = &'a str> + 'a {
+        self.find_all(name).map(Tag::value)
+    }
+
+    /// Whether the collection contains at least one tag with the
+    /// given name.
+    pub fn has(&self, name: &str) -> bool {
+        self.find(name).is_some()
+    }
+}
+
+impl std::ops::Deref for Tags {
+    type Target = [Tag];
+    fn deref(&self) -> &[Tag] {
+        &self.0
+    }
+}
+
+impl From<Vec<Tag>> for Tags {
+    fn from(v: Vec<Tag>) -> Self {
+        Tags(v)
+    }
+}
+
+impl From<Tags> for Vec<Tag> {
+    fn from(t: Tags) -> Self {
+        t.0
+    }
+}
+
+impl FromIterator<Tag> for Tags {
+    fn from_iter<I: IntoIterator<Item = Tag>>(iter: I) -> Self {
+        Tags(iter.into_iter().collect())
+    }
+}
+```
+
+That's the entire query API. No `TagKind`, no marker enum, no parsed
+variants. Readers who want to know whether an event mentions a
+particular pubkey write `event.tags.values("p").any(|p| p == hex)` and
+the code reads the way the question sounds.
+
+## What's next
+
+We now have both halves of an event in their proper types: the
+cryptographic core from the last chapter and the structured data half
+from this one. The next chapter introduces kinds — the integer that
+decides how content and tags on a given event should be read — and
+with it the beginning of a taxonomy we have so far resisted building.

book/plan/tags.md

@@ -0,0 +1,243 @@
+# Plan: Tags
+
+## Topic Summary
+
+Introduce a proper `Tag` type to replace the `Vec<Vec<String>>` used in the
+events chapter. Tags carry the structured data half of every event:
+references to pubkeys, events, addresses, topics, relay hints, and anything
+else machine-readable. This chapter defines a thin wrapper around
+`Vec<String>`, a small set of accessors, free helper functions that query
+slices of tags by name, and an `Address` type for parsing and constructing
+the `kind:pubkey:identifier` form used by `a` tags. It then retrofits the
+existing `Event` pipeline to hold `Vec<Tag>` instead of `Vec<Vec<String>>`.
+
+The chapter stays neutral on tag semantics — no `TagKind` enum, no marker
+parsing. The philosophy (from building-nostr) is clear: the meaning of a
+tag depends on the event's kind, and a generic type that tries to know
+better is almost always wrong.
+
+## Chapter Outline
+
+1. **Opening framing.** Tags are the structured half of events. Lists of
+   lists of strings, not maps, because ordering matters and keys repeat.
+   Single-letter tags are indexed by relays and filterable via `#e`, `#p`,
+   etc. Multi-character tags live in the same array but aren't indexed.
+   Point out that tag semantics depend on event kind — this type stays
+   neutral.
+
+2. **The module.** Register `tags` in `lib.rs`, imports, `use` of
+   `PublicKey`.
+
+3. **The `Tag` type.** `pub struct Tag(pub Vec<String>)`, tuple struct with
+   transparent serde. `Tag::new(name, values)` constructor.
+   `impl From<Vec<String>> for Tag` and back. `impl Deref<Target = [String]>`
+   for ergonomic slice access.
+
+4. **Accessors.**
+   - `fn name(&self) -> &str` — first entry, or empty string if empty
+   - `fn value(&self) -> &str` — second entry, or empty string
+   - `fn values(&self) -> &[String]` — everything after the name
+   - `fn get(&self, i: usize) -> Option<&str>`
+   - `fn len(&self) -> usize`, `fn is_empty(&self) -> bool`
+
+5. **Slice helpers.** Free functions that take `&[Tag]`:
+   - `pub fn find<'a>(tags: &'a [Tag], name: &str) -> Option<&'a Tag>`
+   - `pub fn find_all<'a>(tags: &'a [Tag], name: &str) -> impl Iterator<Item = &'a Tag>`
+   - `pub fn value<'a>(tags: &'a [Tag], name: &str) -> Option<&'a str>`
+   - `pub fn values<'a>(tags: &'a [Tag], name: &str) -> impl Iterator<Item = &'a str>`
+   - `pub fn has(tags: &[Tag], name: &str) -> bool`
+
+6. **A word on markers.** NIP-10 puts thread markers at `tag[3]` on `e`
+   tags. Show how to read them with `tag.get(3)` without introducing a
+   typed marker. Note that reply-thread parsing is kind-aware and belongs
+   in a later chapter.
+
+7. **The `Address` type.** For `a` tags:
+   ```
+   "30023:npub...:my-article-slug"
+   ```
+   - Struct: `kind: u16`, `pubkey: PublicKey`, `identifier: String`
+   - `Address::from_str` / `Display` — splits on `:`, validates kind and
+     pubkey hex, preserves identifier as-is
+   - `Address::to_tag(&self) -> Tag` — emits `["a", "kind:pubkey:d"]`
+   - `Address::from_tag(&Tag) -> Option<Address>` — reads index 1 of an
+     `a` tag and parses it
+   - `AddressError` enum with `InvalidFormat`, `InvalidKind`,
+     `InvalidPubkey`
+
+8. **Retrofit `Event`.** The events chapter holds tags as
+   `Vec<Vec<String>>`. Replace that everywhere with `Vec<Tag>`. The
+   canonical JSON produced by `Sha256::digest(canonical(...))` must remain
+   byte-identical — `#[serde(transparent)]` on `Tag` guarantees this
+   because the canonical form goes through `serde_json::json!`, which
+   sees each `Tag` as its inner `Vec<String>`. Update all six structs in
+   the pipeline, the canonical helper, the `Visitor::visit_map`, and the
+   tests.
+
+9. **Worked example.** Build an event with a few typed tags:
+   ```rust
+   let tags = vec![
+       Tag::new("t", ["nostr"]),
+       Tag::new("p", [pubkey.to_hex()]),
+       Address { kind: 30023, pubkey, identifier: "slug".into() }.to_tag(),
+   ];
+   ```
+   Then show how to read them back with `tags::value(&event.tags, "t")`
+   etc. This is illustrative prose, not tangled.
+
+10. **What's next.** Pointer toward kinds: the type that interprets what
+    a given tag collection *means* in the context of a particular event.
+
+## API Design
+
+New in `coracle-lib/src/tags.rs`:
+
+```rust
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(transparent)]
+pub struct Tag(pub Vec<String>);
+
+impl Tag {
+    pub fn new<N, V, S>(name: N, values: V) -> Self
+    where N: Into<String>, V: IntoIterator<Item = S>, S: Into<String>;
+    pub fn name(&self) -> &str;
+    pub fn value(&self) -> &str;
+    pub fn values(&self) -> &[String];
+    pub fn get(&self, i: usize) -> Option<&str>;
+    pub fn len(&self) -> usize;
+    pub fn is_empty(&self) -> bool;
+    pub fn as_slice(&self) -> &[String];
+}
+
+impl From<Vec<String>> for Tag { ... }
+impl From<Tag> for Vec<String> { ... }
+impl std::ops::Deref for Tag { type Target = [String]; ... }
+
+pub fn find<'a>(tags: &'a [Tag], name: &str) -> Option<&'a Tag>;
+pub fn find_all<'a>(tags: &'a [Tag], name: &str)
+    -> impl Iterator<Item = &'a Tag>;
+pub fn value<'a>(tags: &'a [Tag], name: &str) -> Option<&'a str>;
+pub fn values<'a>(tags: &'a [Tag], name: &str)
+    -> impl Iterator<Item = &'a str>;
+pub fn has(tags: &[Tag], name: &str) -> bool;
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum AddressError {
+    InvalidFormat,
+    InvalidKind,
+    InvalidPubkey,
+    NotAnAddressTag,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct Address {
+    pub kind: u16,
+    pub pubkey: PublicKey,
+    pub identifier: String,
+}
+
+impl Address {
+    pub fn to_tag(&self) -> Tag;
+    pub fn from_tag(tag: &Tag) -> Result<Self, AddressError>;
+}
+
+impl std::str::FromStr for Address { ... }
+impl std::fmt::Display for Address { ... }
+```
+
+## Changes to `events.rs`
+
+- `use crate::tags::Tag;` at the top.
+- Every `tags: Vec<Vec<String>>` becomes `tags: Vec<Tag>`.
+- The `canonical()` helper accepts `tags: &[Tag]` and still serializes
+  identically (Tag is `#[serde(transparent)]`).
+- `Visitor::visit_map`'s `tags: Option<Vec<Vec<String>>>` becomes
+  `tags: Option<Vec<Tag>>`. Same serde behavior.
+- The worked example in prose should use `Tag::new(...)`.
+
+## Code Organization
+
+- `coracle-lib/src/tags.rs` — new file, tangled from the chapter.
+- `coracle-lib/src/lib.rs` — append `pub mod tags;` via a small
+  block in tags.md.
+- `coracle-lib/src/events.rs` — existing file, modified by editing
+  `book/04-events.md` where its blocks are tangled.
+- `coracle-lib/tests/tags.rs` — new hand-written integration tests.
+- `coracle-lib/tests/events.rs` — updated to use `Tag::new` in
+  fixtures instead of `vec!["t".into(), "nostr".into()]`.
+
+## Dependencies
+
+All already present. No new crates.
+
+## Narrative Notes
+
+- **Lead with the philosophy, not the code.** Building-nostr's framing is
+  precise: lists of lists, ordering matters, data tags vs filter tags vs
+  behavior tags got conflated, single-letter tags are indexed. Putting
+  this first makes the eventual tiny type feel justified.
+- **Justify the thinness.** Readers coming from rust-nostr may expect a
+  huge enum. Spell out why we don't: tag meaning is kind-dependent, new
+  tags appear constantly, and a `Custom` catch-all variant is a sign the
+  abstraction is in the wrong place.
+- **Show retrofitting as routine.** Don't hide the fact that we're
+  revising `events.rs`. Literate programs are allowed to grow backward —
+  that's part of why they're literate.
+- **Address gets special treatment.** It's the one tag shape worth a
+  struct: three fields that always appear together and can be validated
+  at parse time. Contrast with `e`/`p`/`t` where the payload is just a
+  single string and a struct would be gratuitous.
+
+## Design Decisions
+
+1. **Tuple struct, not named field.** Matches `PublicKey`/`SecretKey` from
+   chapter 02. Reads naturally and allows direct destructuring.
+
+2. **`#[serde(transparent)]`.** The wire format is unchanged. Canonical
+   hash bytes are unchanged. Prior events round-trip byte-for-byte.
+
+3. **Deref to `[String]`.** Gives iter, len, get for free without a
+   pile of forwarding methods.
+
+4. **`name()`/`value()` return `&str`, never `Option`.** Empty tags would
+   be a protocol error. Returning `""` when the slice is empty keeps call
+   sites simple and matches how every reference implementation reads
+   tags.
+
+5. **Free functions, not methods on a `Tags` newtype.** A `Tags(Vec<Tag>)`
+   wrapper would force users to convert back and forth for serde. Free
+   functions over slices compose with whatever container the caller
+   holds.
+
+6. **No `TagKind` enum.** Single tag names stay as bare `&str`. Building
+   an enum would bake in the semantic-per-kind decision we explicitly
+   reject.
+
+7. **No marker type.** NIP-10 markers live at a positional index and
+   their meaning depends on the event kind. Reply/thread parsing belongs
+   in a reply chapter.
+
+8. **`Address` takes `u16` for kind, matching `Event::kind`.** Addresses
+   in the wild occasionally use kinds beyond 65535 — we match the event
+   type anyway for consistency and revisit only if it bites.
+
+9. **`Address::from_tag` returns `Result`, not `Option`.** Distinguishing
+   "not an `a` tag" from "malformed `a` tag" is useful for error
+   messages and matches our existing error-enum style.
+
+10. **No `relay` field on `Address`.** The third element of an `a` tag is
+    a relay hint, not part of the address. Relay hints are a concept we
+    introduce with relay selections later; folding them in here would be
+    premature.
+
+## Open Questions
+
+- **Should `Tag::new` take a slice literal instead of `IntoIterator`?**
+  `Tag::new("t", ["nostr"])` reads well with `IntoIterator` because
+  array literals satisfy it. Keeping `IntoIterator` to allow passing
+  iterators directly from callers.
+
+- **Should we add `event.tag(name)` / `event.tag_value(name)` methods on
+  `Event`?** Tempting, but method clutter on `Event` grows fast. Sticking
+  to free functions in `tags::` that take `&event.tags`. Revisit if
+  ergonomics suffer in later chapters.

book/research/tags.md

@@ -0,0 +1,248 @@
+# Research: Tags
+
+## Topic Summary
+
+The tags chapter introduces a typed representation of nostr tags to replace
+the `Vec<Vec<String>>` used in the events chapter. Tags are arrays of
+strings whose first element names the tag and whose subsequent elements
+carry values, relay hints, and markers. The chapter should cover:
+
+- A `Tag` wrapper around `Vec<String>` with accessors for name, value, and
+  the rest of the entries
+- Helpers that read and filter tags on an event (`find`, `find_all`,
+  `values`, `value`, `has`)
+- The distinction between indexed single-letter tags and multi-character
+  tags
+- Parsing and constructing address tags (`kind:pubkey:identifier`) and
+  `EventPointer`/`ProfilePointer`/`AddressPointer` conveniences
+- NIP-10 markers on e-tags (`root`, `reply`, `mention`) and how to read
+  them positionally
+- Integration with the `Event`/`EventContent`/etc. types from the events
+  chapter — swap `Vec<Vec<String>>` for `Vec<Tag>`
+
+We want an ergonomic but minimal type. Not rust-nostr's 60-variant enum; a
+thin wrapper plus free functions on slices, close in spirit to nostrlib or
+welshman.
+
+## Philosophy
+
+From `ref/building-nostr`:
+
+**Tags are the structured data half of events.** An event's content is
+generally human-readable; tags hold structured data. Encoding JSON into
+content is an antipattern. Conversely, tags are where every reference,
+index, or machine-readable annotation should live.
+
+**Lists of lists, not maps.** Tags are arrays of arrays of strings by
+design. This preserves two properties a dictionary cannot: keys may repeat
+(important for multiple `e` or `p` references), and order is preserved.
+The parallel drawn by building-nostr is to URL query parameters and Python
+ordered dicts.
+
+**Keep tags short.** "In general, tags should be as short as is reasonable.
+Two to three entries is all you really need; if you have more than that,
+you're probably trying to pack more data into a single tag than really
+belongs." Prefer multiple tags over positional fields.
+
+**Three categories, conflated.** Building-nostr identifies three
+categories of tag that were conflated in the original design: data tags
+(for display/handling), filter tags (single-letter, queryable via `#x`),
+and behavior tags (like `expiration`, `-`, `h` — affect implementation
+handling orthogonally to kind). The conflation is called out as "a design
+mistake" but we have to live with it.
+
+**Single-letter = indexed.** Single-letter tag names (`a`–`z`, `A`–`Z`)
+are the ones relays index and expose via `#e`, `#p`, etc. filters.
+Multi-character names (`imeta`, `alt`, `expiration`) are typically not
+indexed. The tag-name convention is therefore meaningful: naming a tag
+with a single letter asserts it's intended for filtering.
+
+**The `e` tag is overloaded.** Eight different NIPs use `e` for different
+things (reply, fork, transaction reference, report target, list member,
+approval, merge, mention). Building-nostr warns: when resolving a tag's
+meaning, always consult the kind spec first, then tag specs — never the
+other way around. Our library should stay neutral about semantics and let
+callers interpret based on kind.
+
+**Design general-purpose tags cautiously.** Broad tags can conflict with
+kind-specific semantics. Our tag type should not bake in interpretation.
+
+## Reference Implementation Analysis
+
+### applesauce (TypeScript)
+
+- Tags remain `string[][]` throughout; no wrapper class.
+- Type-level annotation via `NameValueTag<Name>` generic tuple; runtime
+  type guards (`isETag`, `isPTag`, ...) identify kinds.
+- Markers (`"root" | "reply" | "mention" | ""`) as a union type.
+- A-tag parsing lives in `parseReplaceableAddress(address)` returning
+  `AddressPointer | null`, with an inverse
+  `getReplaceableAddressFromPointer`.
+- Operations-as-functions: `TagOperation = (tags) => tags`. Events expose
+  `modifyPublicTags(...ops)` that pipes operations.
+- Helpers: `addEventPointerTag`, `addProfilePointerTag`,
+  `addAddressPointerTag`, `ensureSingletonTag`, `ensureNamedValueTag`,
+  `fillAndTrimTag` (normalizes nulls and trailing blanks).
+
+### ndk (TypeScript)
+
+- `NDKTag = string[]`, raw; `NDKEvent.tags: NDKTag[]`.
+- Accessors on event: `getMatchingTags(name, marker?)`, `hasTag`,
+  `tagValue` (returns index 1 or undefined), plus `removeTag`, `replaceTag`.
+- Address tags: `tagAddress()` constructs `${kind}:${pubkey}:${dTag}`;
+  `tagId()` returns event id or address depending on replaceability;
+  `tagType()` returns `"e" | "a"`.
+- NIP-10 markers at `tag[3]`: `getRootTag`, `getReplyTag` fall back to
+  positional interpretation when markers are absent.
+- `referenceTags(marker?)` emits `[["a", addr], ["e", id, relay, marker, pubkey]]`.
+- `generateContentTags` auto-tags `npub`/`note`/`nevent`/`naddr`/hashtags
+  from content.
+
+### nostr-gadgets (TypeScript, JSR)
+
+- Raw `string[]` tags, documented by convention.
+- Single helper: `getTagOr(event, tagName, dflt)`.
+- Validators: `isHex32`, `isATag` (regex `^\d+:[0-9a-f]{64}:[^:]+$`).
+- Composition pattern: `itemsFromTags<I>(processor)` factory — each
+  fetcher passes a per-tag processor to build typed items.
+- Deletion kind-5: switch on `tag[0]` for `e` (id filter) vs `a`
+  (kind+author+#d filter).
+
+### nostrlib (Go, fiatjaf)
+
+- `Tag = []string`, `Tags = []Tag`; embedded directly in `Event`.
+- Helpers on `Tags`:
+  - `Find(key)`, `FindLast(key)`
+  - `FindWithValue(key, value)`, `FindLastWithValue`
+  - `FindAll(key)` returns `iter.Seq[Tag]` (lazy)
+  - `Has(key)`, `ContainsAny(key, values)`
+  - `GetD()` for the `d` identifier on parameterized replaceables
+- Pointer interface: `ProfilePointer`, `EventPointer`, `EntityPointer`
+  all share `AsTag`, `AsTagReference`, `AsFilter`, `MatchesEvent`.
+- Address parsing: `ParseAddrString("kind:pubkey:d")` splits on `:`,
+  validates kind (0..65535) and pubkey (hex), preserves identifier.
+- Standard library only (`iter`, `slices`, `strconv`). No tag taxonomy
+  enum; NIPs implement their own parsing helpers over raw slices.
+- Thread markers (`root`/`reply`/`mention`) and relay-list markers
+  (`read`/`write`) are read via index, never via typed fields.
+
+### nostr-tools (TypeScript)
+
+- Plain `tags: string[][]`, no wrapper.
+- Direct indexing throughout: `tag[0]` name, `tag[1]` value, `tag[2]`
+  relay, `tag[3]` marker, `tag[4]` pubkey hint.
+- Address-tag parsing inline per NIP:
+  `let [kind, pubkey, identifier] = tag[1].split(':')`.
+- NIP-10 supports both explicit markers and legacy positional fallback
+  (oldest/newest heuristic).
+- Each NIP module owns its own tag construction and parsing; no central
+  tag API.
+
+### rust-nostr (Rust)
+
+- `Tag` wraps `Vec<String>` plus `OnceCell<Option<TagStandard>>` for
+  lazy parsed enum.
+- `TagStandard` enum has 60+ variants covering most NIPs (`Event`,
+  `PublicKey`, `Coordinate`, `Kind`, `Amount`, `Image`, `Title`, ...).
+- `TagKind<'a>` categorizes: named variants, `SingleLetter(SingleLetterTag)`
+  with case tracking, `Custom(Cow<'a, str>)`.
+- E-tag parser is position-aware: `tag[3]` attempts Marker first, falls
+  back to PublicKey (NIP-01 legacy); `tag[4]` is PublicKey only if `[3]`
+  was a marker.
+- A-tag parser uses `Coordinate::from_str`.
+- `Tags` collection (not `Vec<Tag>`) maintains a
+  `BTreeMap<SingleLetterTag, BTreeSet<String>>` index for dedup and
+  indexed lookup, plus helpers `event_ids()`, `public_keys()`,
+  `coordinates()`.
+- Trade-offs: extensibility (every new tag type touches the enum),
+  OnceCell overhead per tag, case-preservation fields. Very thorough
+  but heavy.
+- **We should not replicate the enum approach.** Prefer a thin wrapper
+  over `Vec<String>` and let callers parse.
+
+### welshman (TypeScript — predecessor of this library)
+
+- No wrapper class; raw `string[][]`.
+- 50+ pure functions in `/util/src/Tags.ts`:
+  - Filters: `getTags(tagName, tags)`, `getTag(tagName, tags)`
+  - Value extractors: `getTagValues`, `getTagValue`
+  - Type-specific: `getEventTags`, `getPubkeyTags`, `getAddressTags`,
+    `getRelayTags`, `getTopicTags`, `getKindTags`
+  - Reply logic: `getReplyTags`, `getCommentTags` (NIP-10 + NIP-22
+    uppercase/lowercase dual-tag)
+  - `uniqTags` dedup, `tagger` factory
+- Dedicated `Address` class with `kind`, `pubkey`, `identifier`, `relays`;
+  factories `from`, `fromNaddr`, `fromEvent`; `isAddress` regex
+  `^\d+:\w+:.*$`; `toString` and `toNaddr`.
+- Event envelope types (`EventContent`, `EventTemplate`, `StampedEvent`,
+  ...) match our exact Rust hierarchy — this is where we borrowed it.
+  Tags stay as `string[][]`.
+- High-level builders in `/app/src/tags.ts`: `tagEventForReply`,
+  `tagEventForComment`, `tagEventForQuote`, `tagEventForReaction`.
+
+## Common Patterns
+
+**Raw lists dominate.** Every library except rust-nostr keeps tags as the
+native string array. The rust-nostr enum is an outlier, and its heaviness
+is visible (extensibility pain, memory overhead).
+
+**Free functions over methods.** Welshman and applesauce both prefer pure
+functions that take tags and return tags or values. Method-on-type
+approaches (ndk) tend to get cluttered.
+
+**Address tags get their own type.** Nostrlib (`EntityPointer`), welshman
+(`Address`), applesauce (`AddressPointer`), rust-nostr (`Coordinate`)
+all introduce a small struct for `kind:pubkey:d`. This is consistently
+the one tag type worth parsing eagerly because it combines three fields
+that are always used together.
+
+**Markers are positional.** No library introduces a `Marker` enum
+dependency that leaks into the base tag type. Marker interpretation
+happens at the reader site (`getReplyTags` etc.), not at construction
+time.
+
+**Single-letter indexing matters for filters.** Nostrlib and rust-nostr
+explicitly model the single-letter vs multi-character distinction.
+Applesauce and welshman rely on convention.
+
+## Considerations for Our Implementation
+
+Given our literate-programming posture and existing style (thin wrappers
+over bytes in `keys`, struct pipelines in `events`), we should:
+
+1. **Introduce `Tag(Vec<String>)` as a tuple wrapper.** Provide `name()`,
+   `value()` (second element or empty), `values()` (all after the first),
+   `get(i)`, `len()`, `as_slice()`, plus `From<Vec<String>>`,
+   `IntoIterator`, `Serialize/Deserialize` that flatten transparently to
+   an array. `new` constructor that takes a name and variadic values.
+
+2. **Free functions on `&[Tag]`.** `find(tags, name)`, `find_all(tags, name)`,
+   `values(tags, name)`, `value(tags, name)`, `has(tags, name)` as
+   standalone helpers. Keep them name-agnostic — `name` is `&str`.
+
+3. **An `Address` struct for `a` tags.** Fields `kind: u16`,
+   `pubkey: PublicKey`, `identifier: String`, plus optional `relays`.
+   Implement `FromStr`/`Display` for the `kind:pubkey:d` form, and an
+   `Address::to_tag()` / `Address::from_tag()` pair. Keep it minimal —
+   no `naddr` yet (that lands in the bech32/entities chapter).
+
+4. **Update `Event` and friends to use `Vec<Tag>`.** The events chapter
+   left tags as `Vec<Vec<String>>` explicitly because the `Tag` type
+   wasn't ready. Swap it now and keep the canonical hash bytes identical
+   (serialize `Tag` transparently as `Vec<String>` in the canonical form).
+
+5. **Stay neutral on semantics.** No `TagKind` enum, no marker parsing
+   baked into `Tag`. Building-nostr is explicit that tags must be
+   interpreted in the context of the event kind; a generic type should
+   not try to know better.
+
+6. **Brief section on markers.** Show how to read NIP-10 markers
+   positionally — `tag.get(3)` — without introducing a marker type. The
+   marker-aware reply threading will belong in a later chapter.
+
+7. **No hidden-tag / modify pipelines.** That belongs later, with
+   encryption of private tag lists.
+
+The goal is a type that disappears when you're not using it and becomes
+helpful the moment you are — exactly the "little more than an empty
+shell" that building-nostr describes for nostr itself.